NSIS Error: Error launching installer

[Irish2007]

I have an installer that asks the user if they wish to uninstall the previous version of this application. When they click "Yes" they get the popup error "NSIS Error: Error launching installer". There is no other information. If they click OK the uninstaller continues without any problems.

If they run the uninstaller from the shortcut in the start menu, there is no error and the uninstaller runs correctly.

The error only appears when the installer calls the uninstaller.

I read the uninstaller for the last version from the registry:
ReadRegStr $R0 HKLM \
"Software\Microsoft\Windows\CurrentVersion\Uninstall\Application\
"UninstallString"
StrCmp $R0 "" done

Then I run it:
ExecWait '$R0 _?=${PreviousInstLocation}'

I took this from sample code found on the forums and the sourceforge site...

Has anyone seen something similar to this before? As I said, the uninstaller works without error when run from the start menu shortcut; it is only when the installer calls the uninstaller that the error appears.

Thanks for any help you can give me...